Calcium phosphates form a group of biocompatible and biologically active materials almost identical in composition and structure to inorganic substances occurring in vertebrate hard tissues such as bones and teeth.
Among them, hydroxyapatite has properties such that even when implanted in living organisms, it will not cause any rejection reaction or necrosis of the organisms but can readily be assimilated into or directly bound to biological hard tissues; therefore, it is expected to serve as a material for repairing bone defects and bone cavities. While the material forms of hydroxyapatite include compact, porous, granular and cement-like forms, among others, apatite cement which can be molded into any arbitrary shapes is a material having great possibilities in the future.
However, the conventional apatite cements require a long period for hardening and it is also known that the ossification induction period from implantation into living bodies to initiation of assimilation into and conjugation with biological hard tissues is as long as 4 to 5 weeks. This characteristic is associated with patient's pain and is regarded as one of the disadvantages of the currently available apatite cement (Patent Document 1). Further, the conventional apatite cements have a drawback in that they are weak in bending strength (Non-Patent Document 1). A further problem with the conventional apatite cement is that acid-base reactions occur on the occasion of hardening and result in local pH changes until the time of hardening in vivo, inducing inflammatory responses.
Porous bodies comprising β-tricalcium phosphate are currently used as filling materials for transplant bone collection sites or after tumor excision. However, the technology of applying them to wide-ranging defects in long tubular bones supporting a heavy load such as femurs, tibias and the like has not been established as yet. This is because a bone-binding ability sufficient for enduring the excessive stress generated on the interface between a loaded long tubular bone and an artificial bone cannot be obtained in a short period of time. Porous bodies comprising β-tricalcium phosphate are gradually substituted by biological bone but the time required therefor is long; therefore, in real therapy, it is difficult to apply it to loaded parts or sites without using any other fixing material (Non-Patent Document 2). Since β-tricalcium phosphate can characteristically be substituted by biological bone, the development of a material for cement comprising β-tricalcium phosphate is demanded from the clinical side. However, any bioabsorbable cement based on β-tricalcium phosphate alone has not been developed as yet.
One of the present inventors has previously proposed “a chelation-hardening type cement for bone restoration” for solving the problems mentioned above, namely a cement in which the chelation hardening ability of an inositol phosphate is utilized and which comprises one single component capable of hardening without causing changes in pH on the occasion of hardening (Patent Document 2). Inositol phosphates occur in vivo in animals and plants and are very highly safe substances and further are comparable in chelating ability to EDTA.
This cement is expected to be widely applicable as an injectable bone filling material in the fields of orthopedics and dentistry. Since, however, the compression strength provided thereby is 6 to 7 MPa and therefore there is room for investigation from the mechanical strength viewpoint, there is still a problem about the application thereof to sites required to support a high load of at least 14 MPa (e.g. in the case of spinal column compression fracture).    (Patent Document 1) Japanese Patent Laid-Open Publication No. Hei 5-229807.    (Patent Document 2) Japanese Patent Laid-Open Publication No. 2005-95346.    (Non-Patent Document 1) Takafumi Kanazawa: “Rin (Phosphorus)”, pages 65-86 (Kenseisha, 1997).    (Non-Patent Document 2) The Chemical Society of Japan (ed.): “Kagaku Binran Oyokagakuhen II (Manual of Chemistry, Applied Chemistry Section II), 6th edition”, page 1485 (Maruzen, 2003).